In the field of crystalline silicon solar cell, silicon substrates have been reduced in thickness in order to reduce the amount of silicon usage and to improve the conversion efficiency of silicon substrates. Unfortunately, thinner silicon substrates have remarkably lower conversion efficiency. This is because, for example, a large number of defects in the front surface of the silicon substrate having a conductivity mainly cause the reduction in the lifetime of minority carriers (for example, electrons in a p-type substrate) generated by irradiation with light. Thus, reducing the loss of minority carriers eventually improves the conversion efficiency of solar cells.
To regulate the reduction in the lifetime of carries, a passivation film is generally formed on the back surface of the silicon substrate. An aluminum oxide film, one of a plurality of kinds of passivation films, has received attention as the above-described passivation film because of its higher passivation effect (the function to regulate the reduction in lifetime) on the p-type silicon substrate.
The aluminum oxide films include negative fixed charges and are known to produce the passivation effect resulting from the field effect caused by the fixed charge. Thus, the aluminum oxide film including negative fixed charges is formed on the front surface of the p-type silicon substrate to regulate the diffusion of electrons being minority carriers into the surface of the substrate, thereby preventing the loss of carriers.
The aluminum oxide film being the passivation film is formed on the p-type silicon substrate by chemical vapor deposition (CVD) (see, for example, Patent Document 1).